Model thermoregulation, tolerance, ultrafiltration, selective reabsorption, counter-current multiplication, ADH and cardiovascular feedback.
Estimated time: 185 minutes
IB syllabus: D3.3 · SL and HL
Homeostasis maintains internal variables within ranges compatible with cell function despite external change and metabolic disturbance. It is dynamic, not a motionless equilibrium. Blood glucose, temperature, carbon dioxide, pH, osmotic concentration and pressure fluctuate around regulated ranges. Different individuals and species have different tolerance limits, and acclimatization can shift performance within genetically constrained boundaries.
A negative-feedback loop contains a disturbance, sensor, coordination pathway and effector response that opposes the disturbance. A thermostat analogy is incomplete unless the biological parts are identified. Thermoreceptors in skin and hypothalamus detect temperature; the hypothalamus integrates inputs; autonomic nerves, endocrine signals and behavior alter heat production or transfer. The response reduces error but may overshoot or oscillate because sensing and action take time.
When body temperature rises, skin arterioles dilate, increasing blood flow near the surface, and sweat secretion increases evaporative cooling. Hairs lie flatter, and behavior may seek shade or reduce activity. In cold conditions, skin arterioles constrict, skeletal muscles shiver, metabolic heat production rises and behavior adds insulation. Vasodilation does not itself cool the blood; it increases transfer to an environment that must be cooler than the skin. Evaporation remains effective even when air temperature is high if humidity permits.
Positive feedback instead reinforces a change and requires a terminating event. Oxytocin during birth and the mid-cycle estrogen–LH interaction are examples. Feed-forward control anticipates a disturbance, such as increased heart rate at the start of exercise. Classifying a loop requires following the effect of the response on the original variable, not memorizing whether a named hormone is always positive or negative.
Each kidney contains many nephrons. Blood enters a glomerulus through an afferent arteriole and leaves by a narrower efferent arteriole, sustaining high hydrostatic pressure. Water and small solutes pass through fenestrated capillary endothelium, a basement membrane and filtration slits into Bowman's capsule. Cells and most large proteins remain in blood. Ultrafiltration is size- and charge-selective but is not regulated separately for every useful small molecule.
The proximal convoluted tubule reabsorbs all glucose and amino acids under normal conditions, most sodium and much water. Basolateral sodium–potassium pumps create a sodium gradient that drives sodium-coupled uptake at the apical membrane; solutes move into tissue fluid and capillaries, and water follows osmotically. Microvilli increase surface area and mitochondria supply ATP. Glucose in urine can occur when filtered load exceeds transporter capacity, as in poorly controlled diabetes mellitus.
The descending limb is permeable to water but has limited salt permeability, so water leaves as filtrate descends into increasingly concentrated medullary tissue. The ascending limb is impermeable to water; sodium and chloride leave, including active transport in the thick ascending region. Because fluid flows in opposite directions through adjacent limbs and transport is repeated along their length, a modest local difference is multiplied into a steep corticomedullary gradient.
The loop does not simply pump water into the medulla. It separates salt movement from water movement to create the gradient that later permits collecting-duct reabsorption. Blood in the vasa recta exchanges water and solutes counter-currently, carrying away reabsorbed water without rapidly washing out the gradient. Longer loops extend deeper into concentrated medulla and support production of more concentrated urine, an adaptation common in animals from arid habitats.
Osmoreceptors in the hypothalamus respond when blood becomes too concentrated. Neurons synthesize antidiuretic hormone, which is released from the posterior pituitary. ADH binds receptors on collecting-duct cells and triggers insertion of aquaporin channels into the apical membrane. Water then follows the medullary gradient into tissue fluid and blood, producing a smaller volume of concentrated urine. When blood is dilute, ADH release falls, aquaporins are removed and dilute urine is excreted.
ADH conserves water already present; it cannot replace water that has been lost. Alcohol suppresses ADH and can increase urine output, while dehydration normally increases it. The distal tubule and collecting duct also adjust ions and acid–base balance under hormonal control. Urine composition is therefore the result of filtration minus reabsorption plus secretion, not a direct sample of unwanted substances separated at the glomerulus.
Vary ADH and loop length while following water, salt and filtrate through the nephron under hydration and dehydration.
Structure · gradient · exchange · feedback
Baroreceptors in the carotid sinus and aortic arch detect stretch related to arterial pressure and signal cardiovascular centers in the medulla. When pressure falls, sympathetic output can increase heart rate and contractility and constrict selected vessels; opposing parasympathetic effects slow the heart when pressure rises. Chemoreceptor responses to carbon dioxide, pH and low oxygen coordinate breathing with circulation.
Adrenaline and noradrenaline prepare the body for rapid action by increasing cardiac output, redistributing blood, dilating airways and mobilizing glucose. The kidney contributes to longer-term blood-pressure regulation by controlling sodium and water balance and by endocrine pathways. Melatonin released by the pineal gland in darkness helps align sleep timing with the light–dark cycle. These examples show that homeostasis is distributed: hypothalamus, brainstem, pituitary, adrenal glands, kidneys, heart, vessels and behavior cooperate.
A person is dehydrated but has collecting-duct cells that cannot insert aquaporins in response to ADH. Which pattern is most likely?
Practice focused questions or see how IB combines this topic with ideas from elsewhere in the course.
Matching part: 10(c)
Matching part: 39